1. Field of the Invention
The present invention relates to a method for treating a surface of diamond thin film, a method for forming a transistor, and a sensor device.
Priority is claimed on Japanese Patent Application No. 2013-175059, filed Aug. 26, 2013 and Japanese Patent Application No. 2014-138972, filed Jul. 4, 2014, the contents of which are incorporated herein by reference.
2. Description of Related Art
Research and development of sensor devices for detecting a specific material included in a solution (an electrolyte solution), such as ion sensors for detecting an ion concentration in a solution, and biosensors for detecting organic materials including protein materials, glucose, and the like in a solution, has been actively performed. One of the sensor devices includes a field-effect transistor where a source electrode and a drain electrode are formed on a diamond thin film and the surface of diamond thin film positioned between the source electrode and the drain electrode, with which the solution is in contact, acts as a gate. Since the field-effect transistor includes the diamond thin film being in contact with the solution, the field-effect transistor has the advantages that the stability is high, the forming is easy, and the cost is low.
Japanese Unexamined Patent Application, First Publication No. 2012-168120 (referred to as PTL 1) discloses an ion sensor including the field-effect transistor described above. Specifically, PTL1 discloses an ion sensor including a reference electrode and a working electrode. A detection target solution is held between the reference electrode and the working electrode, and each of the reference electrode and the working electrode are configured by the field-effect transistor described above (a p-channel field-effect transistor). Additionally, PTL 1 discloses that the ion sensitivity is controlled by performing a hydrogen-termination with respect to the surface of diamond thin film functioning as a gate, and then, performing an oxygen-termination or a fluorine-termination with respect to a part of the surface of a diamond thin film.
Japanese Unexamined Patent Application, First Publication No. 2006-97054 (referred to as PTL 2), Japanese Patent No. 3886922 (referred to as PTL 3), and Japanese Patent No. 4119973 (referred to as PTL 4) disclose techniques for treating a surface of diamond (a fluorine treatment). Specifically, PTL 2 discloses that the life prolongation is achieved by performing a fluorination treatment (a thermal fluorine treatment, an electrolytic fluorine treatment) with respect to a conductive substrate covered with a conductive diamond. PTL 3 discloses that a density of hole is increased by exposing a surface of diamond to plasma including a mixed gas of hydrogen and fluorine sulfide to treat a surface of a diamond substrate. PLT4 discloses that a fluorine functional group is easily introduced by irradiating a solution including a diamond powder and perfluoroazoalkane with ultraviolet to chemically connect the surface of diamond powder with perfluoroazoalkyl group.
As disclosed in PTL1, it is thought that the ion sensitivity of field-effect transistor can be changed at will by terminating a part of the hydrogen-terminated diamond surface with fluorine. However, by studies of the inventors of the present application, when a fluorine treatment is performed with respect to a surface of diamond, it has come to be understood that a fluorocarbon deposited film is formed depending on the processing method.
As is well known, a fluorocarbon is a collective term of an organic compound including carbon-fluorine bond (C—F), and has a property that a chemical reaction tends not to occur and it is stable if the temperature is changed. Therefore, when a part of the hydrogen terminated diamond surface is fluorine terminated, if such a fluorocarbon deposited film is formed on the diamond surface, there are some cases where the property (nature) of the diamond surface becomes different from a primary intended property. Thereby, the ion sensitivity of the field-effect transistor becomes different from a primary intended ion sensitivity, then, there are some cases where sensor devices including a property different from a primary one are formed.
Therefore, in order to form sensor devices including intended properties, it is quite important to control the presence or absence of the deposited film of fluorocarbon when the diamond surface, which acts as a gate of field-effect transistor, is processed. PTLs 2 to 4 described above disclose a technique for treating a diamond surface (a fluorine treatment), but do not mention that the fluorocarbon deposited film is formed when the diamond surface is treated.